You can do this with the Previous function.
Say I have data like this:
And a table based on this, grouped on month
:
I have applied the following expression to one of the textboxes:
=IIf(Previous(Sum(Fields!val.Value), "monthGroup") > Sum(Fields!val.Value)
, "Red"
, "Green")
For me the expression editor showed a syntax error, but it ran fine:
You could adapt this for other formatting requirements.